Your First FastAPI Route

Start with one Python file. Define an async function, decorate it with @app.get, visit it in the browser. This is the atom of every web app.

# The basic concept: a function that returns data

def health():
    return {'status': 'ok'}

print(health())  # {'status': 'ok'}

# FastAPI adds two things:
#   1. @app.get('/path') decorator  ->  registers the function as a route
#   2. async  ->  the function can await I/O without blocking other requests

# Step 1: A FastAPI app with one route
from fastapi import FastAPI
import uvicorn

app = FastAPI(title='OpenBenchML')

@app.get('/health')
async def health():
    return {'status': 'ok'}

if __name__ == '__main__':
    uvicorn.run(app, host='0.0.0.0', port=8000)

# Install FastAPI + uvicorn
pip install fastapi uvicorn

# Run the server
python main.py

# In another terminal (or browser):
curl http://localhost:8000/health
# {"status":"ok"}

# FastAPI also gives you free docs at:
# http://localhost:8000/docs

How it works

Every web app is just functions that return data when a URL is hit. FastAPI's job is to (a) listen for HTTP requests, (b) match the URL to a registered function, (c) call that function, and (d) send the return value back as JSON. The @app.get('/health') decorator is what registers the function — without it, health() is just a regular Python function that nothing calls. The async keyword means the function can yield control to the event loop while waiting for I/O (database, HTTP, file) — so one worker can handle thousands of concurrent requests. In step 1, there's no I/O, so async is just a convention; it becomes essential in step 3 when we add a database.